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JEuropaischesEuropean Patent Office ® Publication number: 0 075 354

Office européen des brevets B1

® EUROPEAN PATENT SPECIFICATION

(§) Dateof publication of patent spécification: 02.10.85 (g) Int. Cl.4: C 01 F 5/38, C 01 C 1/18 (§) Application number: 82201113.6

® Date offiling: 09.09.82

(M) Process for the préparation of magnésium hexahydrate.

(§) Priority: 17.09.81 NL 8104284 @ Proprietor: UNIE VAN KUNSTMESTFABRIEKEN B.V. Maliebaan 81 (43) Date of publication of application: NL-3581 CGUtrecht(NL) 30.03.83 Bulletin 83/13

® Inventor: Willems, Michael Hendrik (45) Publication of the grant of the patent: Nassaustraat 9 02.10.85 Bulletin 85/40 NL-6166 BD Geleen (NL) Inventor: Vermijs, Winfried J.W. Dunantstraat 4 (84) Designated Contracting States: NL-6164GN Geleen (NL) ATBE DE FRGBIT NL SE

(74) Représentative: Roeffen, Wilhelmus Johannes (§) Références cited: Msris ©t s\ GB-A-2 045 735 OCTROOIBUREAU DSM Postbus 9 US-A-1 844 862 NL-6160 MA Geleen (NL) US-A-3173 756 m m co m o o Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall Û. be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been m paid. (Art. 99(1) European patent convention). Courier Press, Leamington Spa, England. Mg(OH)2, obtained by precipitation of magnesiun compounds from sea water with dolomite The invention relates to a process for the Besides 93-96 wt.% oxide, such preparation of hexahydrate by technical-grade product converting magnesium oxide with in contains a considerable amount of impurities, the presence of water. The invention also relates particularly iron and aluminium oxides, and to a process for the preparation of thermally possibly silicon dioxide. To obtain a good stable containing granules suspension and subsequently a rapid chemical using the magnesium nitrate hexahydrate thus conversion, there is advantage in adding the obtained. magnesium oxide in finely divided form to the A process for the preparation of magnesium magnesium nitrate hexahydrate. By preference nitrate hexahydrate starting from magnesium use is made of magnesium oxide product having oxide, nitric acid and water is known from US an average particle size of at most 200 pm, in patent specification 1,844,862. According to this particular at most 100 µm, and a specific surface known process, finely divided magnesium oxide area larger than 12 m2fg. is added to dilute nitric acid whilst stirring, the For obtaining a homogeneous suspension amount added being sufficient for complete con- there further is advantage in having the suspen- version, on which the mixture is allowed to settle, sion process take place while stirring con- the solid matter is filtered off, the remaining tinuously and intensively. solution is evaporated and finally magnesium The amount of magnesium oxide suspended in nitrate hexahydrate is crystallized out by cooling. the magnesium nitrate hexahydrate may be A disadvantage of this known process is that a chosen within wide limits. For economic opera- large proportion of the impurities introduced with tion of the process, however, it is important to the magnesium oxide goes into solution, and make the amount of magnesium oxide subsequently, on cooling, to a large extent suspended in the magnesium nitrate hexahydrate crystallizes out, together with the magnesium as large as possible. Of course, this amount must nitrate hexahydrate formed, so that an impure be smaller than the maximum amount of product is obtained, which, in addition, is not magnesium oxide that is allowable in view of the easily filtered. viscosity of the suspension. In general, such an Another disadvantage of this process is that in amount is added that the magnesium oxide its practical realization always an excess of water concentration of the suspension obtained is 5-25 is introduced, which must subsequently be wt.%. removed through evaporation. According to the invention, subsequently a If this product is used in the preparation of nitric acid solution is added to the resulting thermally stable ammonium nitrate granules, it mixture, which is meanwhile being stirred. The furthermore causes severe fouling of the concentration of the nitric acid solution is chosen evaporators of the ammonium nitrate plant and, if so that practically all water introduced with the prilling is applied, it gives rise to clogging of the nitric acid and formed during the reaction is spray openings of the prilling bucket. bound as crystallization water in the resulting The present invention provides a process in magnesium nitrate hexahydrate. which the abovementioned disadvantages do not Given the gross reaction equation: or hardly occur. According to the invention this is achieved by: a) Suspending magnesium oxide in melted magnesium nitrate hexahydrate having a theoretically this would require an amount of temperature in excess of 90°C; water corresponding to the amount of water in b) Adding to the suspension obtained, whilst nitric acid with a concentration of 58.33 wt.% stirring, a nitric acid solution, resulting in the HN03. For practical purposes use is made of a formation of more magnesium nitrate nitric acid having a slightly lower concentration, hexahydrate; for instance 53-58 wt.%, in particular about 55 c) Filtering the liquid reaction mixture while wt.% HN03, because during the addition of the maintaining the temperature of the reaction acid part of the water introduced evaporates due mixture in steps (b) and (c) in excess of 90°C; to the strongly exothermic reaction that takes d) Discharging as product a part, corresponding place. to the amount of magnesium oxide introduced, of The temperature during the addition of the acid the liquid magnesium nitrate hexahydrate can be chosen within wide limits. At least this obtained as clear filtrate; temperature must be, as in the suspension e) Utilizing the remaining part as liquid medium process, 90°C, for at lower temperatures for the conversion of magnesium oxide with nitric crystallization of the magnesium nitrate hexa- acid. hydrate takes place. In the process according to the invention use Since at a high temperature the magnesium can be made of both a chemically pure and the nitrate hexahydrate decomposes into lower commercially available technical-grade hydrates or basic salts may be formed, the magnesium oxide, for instance obtained bi temperature is by preference kept below 120°C. in calcination of natural magnesite (MgC03) or c view of the large amount of reaction heat that evolves, this will generally necessitate cooling of nitrate-containing solution or melt, following the mixture during the addition of the acid. which the mixture obtained is granulated in a way The way in which the acid is added is of great known per se, optionally after evaporation. The importance in the subject process. It has been amount of magnesium nitrate hexahydrate to be found that, on the one hand, if the pH becomes added for this stabilization may vary. In general, too low (locally) because of a too large acid an amount of 0.1-1 %, in particular about 0.5 addition, various impurities, notably iron and wt.%, calculated as magnesium oxide relative to aluminium oxides, dissolve, so that they cannot the amount by weight of ammonium nitrate is be removed by filtration at a later stage, while on added. It has been found that the water content of the other hand, of course, part of the magnesium the final product obtained must be ,0.5 wt.%. oxide is not converted if the amount of acid added This can be achieved in a known way, for instance is insufficient. by evaporation of the liquid mixture to be Since it has been found that the filtration of the granulated or by drying of the granular product. mixture if favourably affected by a small amount It is true that the thermal stabilization of of non-converted magnesium oxide, by ammonium nitrate using magnesium nitrate preference use is made of such an amount of (hydrates) is known per se, see for instance the nitric acid that 92 to 98 wt.%, in particular about British patent specifications 914,606 and 988,095, 95 wt.% of the magnesium oxide introduced is but in these known processes use is made of converted into magnesium nitrate hexahydrate. water-free magnesium nitrate or lower hydrates, To eliminate the problems attaching to a too often in solution, or use is made of a magnesium large acid addition to the largest possible extent, nitrate hydrate prepared in another way than in by preference first such an amount of nitric acid is the subject process. added to the suspension that the pH of the The invention will be elucidated in the following resulting mixture is 6-6.5, this amount being examples. added at such a rate and while stirring so intensively that the reactants are homogeneously Example 1 mixed, so that the pH in the mixture locally To 600 g melted Mg(NO3)2 · 6H2O having a nowhere decreases to below about 4, and temperature of 95°C, an amount of 100 g subsequently, whilst stirring, slowly so much magnesium oxide was added, on which the nitric acid is added until a pH of 5.2-5.8 has been mixture was stirred for 15 minutes. reached in the mixture. This last addition is As magnesium oxide, use was made of a stopped when the pH decrease in the mixture, technical-grade product having an average which initially proceeds proportionally with the particle size of <75 pm and a specific surface area amount of nitric acid added, stops or proceeds of about 50 m2/g which contained 2000 ppm(*) very slowly. Fe203, 1800 ppm AI203, 8000 ppm Si02 and 1.3 The liquid mixture obtained after this acid wt.% CaO. addition is subsequently filtered, care being taken Whilst stirring intensively, a 55 wt.% HN03 that the temperature is kept above 90°C. It has solution was added to the suspension obtained, been found that the mixture has very good the metering rate being such that after 1 hour the filtering properties so that, in contrast with the pH of the mixture was 6.0. While the acid was known processes, the use of special filter aids is being added, the temperature was kept at about superfluous. It has been found that the non- 105°C. converted magnesium oxide is deposited on the Subsequently, such an amount of 55 wt.% filter and as it were covers it with a layer, on HN03 was very slowly added to the mixture, which layer subsequently practically all impurities which was meanwhile being stirred, that the pH of are deposited. the mixture was 5.4 after another hour. The Of the liquid magnesium nitrate hexahydrate temperature was kept at about 95°C. The liquid obtained as clear filtrate a part, corresponding to reaction mixture obtained was filtered, the filtrate the amount of magnesium oxide introduced, is obtained being 1175 g clear, liquid discharged as product, while the remaining part is Mg(NO3)2 · 6H2O with a crystallization tempera- re-used as liquid medium for conversion of ture of 88°C which contained <1 ppm Fe203, magnesium oxide with nitric acid. about 2 ppm A1203 and 1 ppm Si02. The liquid magnesium nitrate hexahydrate Of this filtrate, 575 g was discharged as product, obtained as product can in principle be put to while the remaining 600 g was utilized for a various uses, for instance, as such or after mixing further conversion of Mgo with HN03. with components, as, optionally liquid, fertilizer. The product is also suitable for conver- sion, without further pretreatment, into, for Example 2 instance, lower hydrates. To an NH4N03 solution containing 4 wt.% wt.% The product is pre-eminently suitable for the water, an amount of 2.9 Mg(NO3)2 · 6H2O, of stabilization against thermal crystal modifications obtained according to the process Example 1, of ammonium nitrate-containing granules. was added. The liquid mixture, with a To this end, according to the invention a small temperature of about 160°C, was sideways amount of the liquid magnesium nitrate hexa- hydrate obtained is added to an ammonium (*) ppm: parts per million by weight sprayed into a circular bed (temp. 135°C) of 1-4, characterized in that a nitric acid solution fluidized NH4N03 nuclei (average diameter 2.7 having a concentration of 53-58 wt.% HN03 is mm, bed height 40 cm) by means of a powerful air added to the suspension. stream via a two-phase sprayer. Via an overflow 6. Process according to any one of the claims the granules formed were discharged to a sieve, 1-5, characterized in that the temperature of the and there they were separated into a fine fraction mixture is kept at between 90 and 120°C while the (<2 mm), a product fraction (2-4 mm) and a acid is being added. coarse fraction (>4 mm). The product fraction, 7. Process according to any one of the claims which contained 0.2 wt.% H20 and 0.5 wt.% MgO, 1-6, characterized in that such an amount of a was rapidly cooled to about 35°C in a fluidized bed nitric acid solution is added to the suspension that and subsequently five times heated and cooled 92-98 wt.% of the magnesium oxide is between 15 and 50°C. After 5 cycles the product converted into magnesium nitrate hexahydrate. exhibited practically no reduction of the crushing 8. Pocess according to any one of the claims strength (about 45 bar), while only a very slight 1-7, characterized in that first such an amount of volume increase had occurred (<2%). nitric acid is added to the suspension that the pH of the resulting mixture is 6-6.5, this being done Example 3 at such a rate and whilst stirring so intensively A mixture of 97 parts by weight of a 96 wt.% that the reactants are mixed homogeneously, and NH4N03 solution and 3 parts by weight subsequently nitric acid is slowly added to the Mg(NO3)2 · 6H2O (obtained according to the acidified mixture until a pH of 5.2-5.8 has been process of Example 1) was evaporated to a water reached in the mixture. content of 0.2 wt.% and subsequently sprayed in a tower in counterflow with a cool air stream. The prills obtained were cooled to about 45°C in a fluidized bed and subsequently subjected to 5 1. Verfahren zur Herstellung von Mag- cycles between 15 and 50°C. The product showed nesiumnitrat-hexahydrat durch Umsetzung von no deterioration in crushing strength and hardly Magnesiumoxid mit Salpetersäure in Gegen- any volume increase. wert von Wasser, durch gekennzeichnet, daß man a) Magnesiumoxid in geschmolzenem Mag- nesiumnitrat-hexahydrat mit einer Temperatur von mehr als 90°C suspendiert; 1. Process for the preparation of magnesium b) eine Salpetersäurelösung zu der erhaltenen nitrate hexahydrate by converting magnesium Suspension unter Rühren hinzufügt, wodurch oxide with nitric acid in the presence of water, this mehr Magnesiumnitrathexahydrat gebildet wird; process being characterized in that c) das flüssige Reaktionsgemisch filtriert, a) Magnesium oxide is suspended in melted während man die Temperatur des Reaktions- magnesium nitrate hexahydrate having a gemisches in den Stufen (b) und (c) über 90°C temperature in excess of 90°C; hält, b) A nitric acid solution is added to the d) vom flüssigen, als klares Filtrat erhaltenen suspension obtained, whilst stirring, resulting in Magnesiumnitrat-hexahydrat einen Teil, en- the formation of more magnesium nitrate hexa- tsprechend der Menge an eingebrachtem hydrate; Magmesiumoxid, als Produkt austrägt; c) The liquid reaction mixture is filtered while e) den verbleibenden Teil als flüssiges Medium maintaining the temperature of the reaction für die Umsetzung von Magnesiumoxid mit mixture in steps (b) and (c) in excess of 90QC; Salpetersäure verwendet. d) Of the liquid magnesium nitrate hexahydrate 2. Verfahren nach Anspruch 1, dadurch gekenn- obtained as clear filtrate a part, corresponding to zeichnet, daß ein technisch reines Magnesium- the amount of magnesium oxide introduced, is oxid mit einer durchschnittlichen Teilchengröße discharged as product; von höchstens 200 µm eingesetzt wird. e) The remaining part is utilized as liquid 3. Verfahren nach 1 oder dadurch medium for the conversion of magnesium oxide Anspruch 2, with nitric acid. gekennzeichnet, daß ein Magnesiumoxid mit ein- durchschnittlichen 2. Process according to claim 1, characterized in er Teilchengröße von höch- 100 und einer Oberfläche that use is made of a technical-grade magnesium stens um spezifischen von größer als 12 m2/g eingesetzt wird. oxide having an average particle size of at most 200 4. Verfahren nach einem der Ansprüche 1-3, pm. dadurch daß 5-25 Gew.-% 3. Process according to claim 1 or 2, charac- gekennzeichnet, terized in that use is made of a magnesium oxide Magnesiumoxid im geschmolzenen Mag- nesiumnitrat-hexahydrat suspendiert werden. having an average particle size of at most 100 pm and a specific surface area larger than 12 m2/g. 5. Verfahren nach einem oer Ansprüche 1-4, 4. Process according to any one of the claims dadurch gekennzeichnet, daß die Suspension mit 1-3, characterized in that 5-25 wt.% einer Salpetersäurelösung einer Konzentration magnesium oxide is suspended in the melted von 53-58 Gew.-% HN03 versetzt wird. magnesium nitrate hexahydrate. 6. Verfahren nach einem der Ansprüche 1-5, 5. Process according to any one of the claims dadurch gekennzeichnet, daß während des Zusatzes der Säure die Temperatur des Gemis- e) la partie restante est utilisée comme milieu ches zwischen 90 und 120°C gehalten wird. liquide pour la transformation de l'oxyde de 7. Verfahren nach einem der Ansprüche 1-6, magnésium avec l'acide nitrique. dadurch gekennzeichnet, daß die Suspension mit 2. Procédé selon la revendication 1, caractérisé einer solchen Menge an Salpetersäurelösung en ce qu'on emploie un oxyde de magnésium de versetzt wird, daß 92 bis 98 Gew.-% des qualité industrielle ayant une grosseur moyenne Magnesiumoxids zu Magnesiumnitrat-hexa- des particules au plus égale à 200 I1m. hydrat umgesetzt werden. 3. Procédé selon la revendication 1 ou 2, 8. Verfahren nach einem der Ansprüche 1-7, caractérisé en ce qu'on emploie un oxyde de dadurch gekennzeichnet, daß die Suspension magnésium ayant une grosseur moyenne des zuerst mit einer solchen Menge an Salpetersäure particules au plus égale à 100 pm et une surface versetzt wird, daß der pH-Wert des resultierenden spécifique d'une valeur supérieure à 12m2/g. Gemisches 6 bis 6,5 beträgt, was mit einer 4. Procédé selon l'une quelconque des solchen Geschwindigkeit und unter so intensivem revendications 1 à 3, caractérisé en ce que Rühren erfolgt, daß die Reaktionskomponenten 5-25% d'oxyde de magnésium est mis en homogen vermischt werden, und sodann suspension dans l'hexahydrate de nitrate de Salpetersäure langsam zu dem angesäuerten magnésium fondu. Gemisch hinzugefügt wird, bis ein pH-Wert von 5. Procédé selon l'une quelconque des 5,2 bis 5,8 im Gemisch erreicht ist. revendications 1 à 4, caractérisé en ce qu'on ajoute à la suspension une solution d'acide nitrique ayant une concentration de 53-58% en poids de HN03. 1. Procédé de préparation de l'hexahydrate de 6. Procédé selon l'une quelconque des nitrate de magnésium par transformation d'oxyde revendications 1 à 5, caractérisé en ce que la de magnésium avec de l'acide nitrique en température du mélange est maintenue entre 90 présence d'eau, ce procédé étant caractérisé en ce et 120°C pendant l'addition de l'acide. que 7. Procédé selon l'une quelconque des a) l'oxyde de magnésium est mis en suspension revendications 1 à 6, caractérisé en ce qu'on dans l'hexahydrate de nitrate de magnésium ajoute à la suspension une quantité d'acide fondu ayant une température supérieure à 90°C; nitrique telle que 92-98% en poids de l'oxyde de b) une solution d'acide nitrique est ajoutée à la magnésium soit transformé en hexahydrate de suspension obtenue, sous agitation, ce qui résulte nitrate de magnésium. dans la formation d'une quantité supplémentaire 8. Procédé selon l'une quelconque des d'hexahydrate de nitrate de magnésium; revendications 1 à 7, caractérisé en ce que l'on c) le milieu réactionnel liquide est filtré tout en ajoute d'abord à la suspension une quantité maintenant la température du milieu réactionnel d'acide nitrique telle que le pH du mélange dans les étapes b) et c) supérieure à 90°C; résultant soit de 6-6,5 cette addition étant faite à d) de l'hexahydrate de nitrate de magnésium une vitesse telle et en agitant si intensément que liquide obtenu sous forme de filtrat limpide, une les réactifs soient mélangées de façon homogène, partie, correspondant à la quantité d'oxyde de et que l'on ajoute ensuite lentement au mélange magnésium introduite, est déchargée comme acidifié de l'acide nitrique jusqu'à ce que l'on produit; atteigne un pH de 5,2-5,8 dans le mélange.